1. Field of the Invention
The present invention relates to a substrate treating apparatus, which treats a substrate constituting an image display apparatus, and a substrate treating method. The present invention further relates to a method for manufacturing a high-voltage device.
2. Description of the Related Art
Recently, various types of flat type image display apparatuses have attracted attention as a next-generation light and thin image display apparatus to replace a cathode-ray tube (hereinafter referred to as a CRT). For example, there has been known a plasma display (PDP) using luminescence of a phosphor due to a discharge phenomenon, a liquid crystal display (LCD), and a display using an electron-emitting device such as a filed emission device and a surface-conduction electron-emitting device.
In general, the display using the electron-emitting device uses a rear plate with the electron-emitting device disposed thereon and a face plate with a phosphor layer. When an image is displayed, an anode voltage is applied to the phosphor layer. An electron beam emitted from the electron-emitting device is accelerated by the anode voltage to be collided with the phosphor layer, whereby the phosphor layer emits light to display the image. In order to obtain practical display properties, it is necessary to use a phosphor similar to a normal CRT, and to set the anode voltage to several kV or higher, preferably to 5 kV or higher.
As described above, when a high voltage is applied as the anode voltage, the formation of an intense electric field in a small gap between the face plate and the rear plate cannot be avoided, and therefore, discharge between the both plates (breakdown) becomes a problem. If the discharge occurs, an electric current of 100 A or higher may momentarily flow, whereby an electron-emitting device, a phosphor surface, and a drive circuit may be destructed or deteriorated. The damage due to such a discharge is causative of a fatal defect of products, and thus, a measure for preventing the occurrence of discharge for a long period of time is required. Here, a voltage which can be applied without the occurrence of discharge is called a “withstand voltage”.
As a method for preventing the discharge, there has been known a treating method (hereinafter called a “withstand voltage treatment”) which realizes the improvement of the withstand voltage by the removal of undesired substances remaining on a face plate and a rear plate. For example, in a withstand voltage treating method for a substrate disclosed in Japanese Patent Application Laid-Open No. 2003-303545, a substrate and a treatment electrode are disposed so as to be opposed to each other in a vacuum atmosphere, and an electric field is applied to between the substrate and the treatment electrode to thereby apply the electric field treatment to the substrate. According to this method, undesired substances and protrusions remaining on the substrate are absorbed to the treatment electrode to be then removed, and the factor of the occurrence of discharge can be removed. The image display apparatus is constituted by using this substrate, whereby it is possible to realize the improvement of the withstand voltage property.
In the above withstand voltage treatment, when undesired substances are previously adhered onto the surface of a treatment electrode, or when undesired substances removed from a substrate are deposited on the treatment electrode due to the repetition of the withstand voltage treatment, leading to the reduction of the effect of the withstand voltage treatment, and the withstand voltage required for the substrate cannot be ensured. The reduction of the treatment effect is partly due to the returning of the undesired substances, adhered onto the treatment electrode surface, to the target substrate. Another factor is the reduction of a treatment electric field due to a field emission current generated from the undesired substances adhered to the treatment electrode. For example, when a glass with a volume resistance of 1010 Ω·cm and a thickness of 0.1 cm is used as a dielectric body, if the field emission current of 1 nA with a beam cross section of 10−6 cm2 is emitted from undesired substances adhered onto the surface of the dielectric body, the voltage drop of 10 kV-order occurs in the glass by estimate. According to approximate calculation performed by us, the voltage drop in a glass becomes a problem in a range from 10 nA to 1 μA. The voltage drop in the dielectric body is causative of the reduction of the surface potential in the target substrate at a position opposed to the adhered undesired substances, whereby the treatment effect is reduced.
There is another problem in the withstand voltage treatment in that, when there is a region with a small adhesion force (easily to be fallen and peeled) in a part of the target substrate due to factors in the manufacturing process, even if the target substrate has a poor withstand voltage, the withstand voltage treatment is continued. If the defect in such a substrate cannot be detected, the substrate proceeds to the following process (assembly process and the like) and therefore to lead to a reduction in panel production rate. In addition, if the target substrate has a region with a small adhesion force, a large number of fallen undesired substances are deposited on the treatment electrode to lead to increasing of the frequency of maintenance (replacement or surface cleaning) for the treatment electrode. Thus, it is necessary to immediately stop the withstand voltage treatment to the target substrate which has on its surface a region with a small adhesion force, and the substrate is required to be treated as a defective.